Harmonic-frequency generator



Oct. 11, 1949. R. E. STURM HARMONIC FREQUENCY GENERATOR 2 Sheets-Sheet l l ll i d INVENTOR Filed Sept. 20, 1941 RUDOLF E. STURM.

ATTORNEY Oct. 11, 1949. R. E. STURM HARMONIC FREQUENCY GENERATOR 2 Sheets-Sheet 2 ..Filed Sept. 20, 1941 lmw wd o wu wd INVENTOR RUDOLF E. STURM.

ATTORNEY Patented Oct. 11, 1949 UNITED STATES;

OFFICE HARMONIC-FREQUENCY GENERATOR Application September 20, 1941, Serial No. 411,624

1 Claim. 1

The present invention-relates to harmonicfrequency generators and, more particularly, to

harmonic-frequency generators of the type adapted to develop in an output circuit thereof a Wide range of harmonic components. Such generators have many uses, for example, in frequency-calibrating carrier-signal communication equipment and in multi-channel carrier-signal transmission systems.

Harmonic-frequency generators generally include a piezo-electric crystal-controlled oscillator from the output of which there are derived signal components which are harmonics of the natural frequency of the piezo-electric crystal. In accordance with one prior art arrangement, the harmonic-frequency generator comprises simply a conventional crystal-controlled oscillator so energized and biased that the anode current of the oscillator consists of current pulses occurring at the period of the crystal and of duration short compared to the crystal period. A current of this nature is rich in harmonics of the natural frequency of the oscillator. While this prior art arrangement is fairly simple, it has the disadvantage that the harmonic-frequency components have relatively little energy content and the generator, therefore, may be used only in conjunction with a high gain amplifier which forms a complex and expensive component of the generator or an adjunct thereto.

It has been proposed, in accordance with another prior art arrangement, that a crystalcontrolled oscillator be used to synchronize the operation of a multivibra'tor which develops an output current of substantially rectangular Wave form which, as is Well known, is rich in harmonics of the natural frequency of the multivibrator. The multivibrator is isolated from the crystalcontrolled oscillator and from the output circuit of the generator by buffer amplifiers. Such amplifiers are essential to prevent reaction by the multivibrator on the crystal-controlled oscillator and to prevent the reaction of such circuits and apparatus as are coupled to the output circuit of the generator from affecting the operation of the multivibrator. The prior art arrangements of this nature are, therefore, also unduly complicated and expensive. They have the further disadvantage that it is difficult to construct multivibrators for operation at frequencies higher than about 100 kilocycles, due to the inherent circuit and tube capacitances. Thus, it is diificult to develop by such harmonic generators very high-frequency harmonics having any appreciable power.

It is an object of the present invention, there..- fore, to provide a new and impoved harmonic.- frequency generator which avoids one or morev of the disadvantages and limitations of the prior art devices.

It is another object of the invention to provide a harmonic-frequency generator of simple and inexpensive construction involving a minimum of circuit elements and one which is capable of delivering directly, and without the need of addi tional amplification, harmonic-frequency com-,- ponents having substantial amplitude, for example, of the order of one volt or more.

It is an additional object of the invention to provide a harmonic-frequency generator which not only involves a simple and inexpensive construction, but which has great flexibility in its operating characteristics and is adapted to deliver large amplitude harmonic-frequency components over a wide harmonic-frequency range, for example, one which may extend into the high-frequency range well about 10 megacycles.

In accordance with the invention, a harmonicfrequency generator comprises means for generating primary oscillations of a predetermined fundamental frequency and means for developing secondary oscillations of higher frequency than the primary oscillations. The generator includes means for combining the primary and secondary oscillations to derive oscillations of unsymmetrical wave form having a large peak amplitude of given polarity during only one relatively small portion of each cycle. The generator also includes means for clipping the oscillations of the unsymmetrical Wave form to derive only the peak-amplitude portion effectively comprising a periodic Wave of pulse-wave form, of period equal to that of the primary oscillations, and rich in harmonic-frequency components.

In a preferred form of the invention, a harmanic-frequency generator of the type described comprises means for developing periodically-recurrent damped trains of oscillations and means responsive to the oscillations for deriving a periodic wave effectively of pulse-Wave form, of period equal to that of the recurrent oscillation trains, and rich in harmonic-frequency components. The generator is provided with an output circuit including means for selecting desired ones of the harmonic-frequency components.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, and its scope Will be pointed out in the appended claims.

Referring now to the drawings, Fig. 1 is a circuit diagram representing a complete harmonicfrequency. generator embodying the invention; Fig. 2 is a graph representing current and voltage relationships occurring at selected points in the arrangement of Fig. 1 and is used in explaining the operation of the invention; Fig. 3 is a circuit diagram of a modified form of the fclipper system of Fig. 1; Figs. 4 and 6 are circuit diagrams representing modified forms of harmonic-frequency generators embodying the invention; while Figs. and 7 are graphs representing voltage and current relationships occurring at selected points in the respective arrangements of Figs. 4 and 6.

Referring now more particularly to Fig. -1 of the drawings, there is represented a complete harmonic-frequency generator embodying the present invention in a preferred form. The generator includes means for generating primary oscillations of a predetermined fundamental and. preferably constant frequency, this means being represented generally by an alternating current source l0. Coupled to the source 16 is an oscillator ll of the blocking-oscillator type. This oscillator is of conventional form and includes a vacuum tube [2 having its input and output electrodes coupled by a transformer l3, the input circuit including a grid condenser l4 and an adjustable grid resistor l5 for determining the natural frequency of oscillation. Coupled to the input-electrode circuit of the oscillator II is means for developing secondary oscillations of higher frequency that the primary oscillations of oscillator ll comprising a vacuum tube I6 having an inductor I! included in the anode circuit thereof. The inductor I! is tuned to a frequency higher than that of the primary oscillations of oscillator H by the capacitance represented by the broken-line condenser l8 for the reason that it may be comprised in whole or in part of the distributed capacitance of the inductor l1 and the inherent anode circuit capacitance.

The tuned circuit [1, I8 is damped by the total effective resistance r in shunt thereto, thus to determine the decrement of the train of secondary oscillations developed across this circuit. This effective resistance is shown in dotted lines for the reason that it may be comprised in whole or in part of the inherent resistance of the inductor ll, of physical resistance connected across inductor l1, and of the effective resistance of the rectifier device l9 and the resistor 20.

The generator also includes means responsive jointly to the primary oscillations of the oscillator II and the secondary oscillations developed by the amplifier l6 to derive a periodic potential effectively of pulse-Wave form, of period equal .to that of the primary oscillations, of a duration of the order of one-half the period of the secondary oscillations, and rich in harmonic-frequency components. This means comprises a rectifier ,device 19 and load impedance 28 therefor coupled across the inductor l'l through a resistor 2| included in the anode circuit of amplifier 16, theunidirectional potential drop across the resistor 2i providing a bias for the rectifier device IS.

The periodic potential of pulse-wave form developed across the load impedance 28 is applied to an amplifier 22. The output circuit of amplifier 22 is coupled to the harmonic generator output circuit which includes means for selecting desired ones of the harmonic-frequency components comprising a harmonic selector system 23 which is provided with a plurality of series-tuned circuits 2d, 25, 25 and 2'7, each being tuned to a predetermined selected harmonic frequency. The coupling between the amplifier 22 and the harmonic selecting system 23 comprises a pair of band-pass selectors 28, 29 which are coupled by a link circuit 30 and which have pass bands sufficiently broad to pass all desired harmonicfrequency components. The use of the link circuit 30 permits the use of the two band-pass selectors 28 and 29, as distinguished from the use of only one such selector, whereby the selectivity of the coupling arrangement is increased and the pass band of the tandem-coupled selectors 28 and 29 is no wider than necessary to pass the desired harmonic-frequency components. The several series-tuned circuits 24-21, inclusive, are coupled to the output terminals 3%, M of the generator by a plurality of respective coupling condensers 32-35, inclusive, and a selector switch 36. Additional selectivity is obtained by connecting an inductor 37 across the output terminals 3|, 3| and by adjusting the several condensers 32-35, inclusive, in such manner that each condenser forms with the inductor 31 a series-tuned circuit resonant at the harmonic frequency to which the series-resonant circuit associated with the particular condenser is tuned.

Considering now the operation of the circuit above described and referring to the curves of Fig. 2, the primary oscillations of source II] are assumed to be of sinusoidal wave form, as represented by curve A. These oscillations are applied as synchronizing oscillations to the blocking oscillator ll. As is well known, the periodic potential developed across the resistor l5 in the input circuit of oscillator H is of generally sawtooth wave form having a large negative peak amplitude and a small positive peak amplitude, as represented by curve B, the period of this potential being the same as that of the oscillations of source ll]. Thus, oscillator ll comprises means responsive to the oscillations of the source. in for developing oscillations of distorted wave form and of period equal to that of the oscillations of source Ill. When the potential developed across the resistor I5 is applied to amplifier if, there is developed across the inductor ll in the output circuit thereof periodically-recurrent damped trains of secondary oscillations of frequency equal to the resonant frequency of the inductor l1 and capacitance Hi, this frequency being much higher than that of the primary oscillations of source ii). That is, the rapid rate of increase of the negative potential developed across the resistor l5 during the time intervals t1tz, tst4,

'etc., shock-excites the tuned circuit ll, 18 to de- .the inherent resistance r of the tuned ,circuit cuit l1, l8 may be so critically damped that only one complete oscillation is developedacross the tuned circuit duringeach of the intervals t1-it3, ts-ts, etc. These damped trains of oscillations are applied to the rectifier device H which is so biased by the unidirectional potential across the resistor 2| that it is conductive only during the largest amplitude positive half-cycle of the train of oscillations. This biasing level is represented in Fig. 2 by the broken line a, b. Thus, the rectifier device |9 comprises a clipping system arranged to clip only the peak portion of the first positive half-cycle of each train of oscillations developed across the inductor l1.

There is consequently developed across the rectifier load impedance 20 a periodic potential wave efiectively of pulse-wave form, as represented by curve D. This periodic wave has a period equal to that of the recurrent oscillation trains, which, in turn, is equal to the period of the primary oscillations of source I0, and the pulses thereof have a duration of the order of but usually less than one-half period of the secondary oscillations developed across the inductor H. The periodic potential of curve D is thus rich in harmonic-frequency components and is amplified by the amplifier 22 and applied to the harmonic selector system 23, wherein a desired harmonic-frequency component is selected by operation of the selector switch 36 and applied to the output terminals 3|, ill of the generator.

As illustrative of a specific embodiment of the invention, the following circuit constants are given for an embodiment of the invention of the type shown in Fig. 1:

Vacuum tubes l2 and Hi Type 6N7 Vacuum tube l9 Type 6H6 Transformer I3 Primary winding inductance millihenries 80 Secondary winding inductance do, 136 Mutual inductance do, 40

Resistor l5 (maximum) .o'hms- 20,000 Resistor 2| do 6,000 Resistor 2|! do 3,300 Condenser M micromicrofarads 500 Inductor ll millihenry 1.2 ohms 27 Frequency of source kilocycles 37.5 Frequency of secondary oscillations, (approx.) megacycle 1 Source +B volts 270 From the above description of the Fig. 1 arrangernent, it will be seen that the saw-tooth oscillator l2 and amplifier I6 comprise means coupling the primary oscillation generator or source It! and the secondary oscillation developing means comprising the inductor l1 and capacitance Hi to synchronize the secondary oscillations with the primary oscillations.

Fig. 3 is a circuit diagram of a modified form of clipper system suitable for use in the system of Fig. 1. This clipper system is essentially similar to that of Fig.11, similar circuit elements being designated by similar reference numerals, except that the rectifier device I9 is coupled to the output of the amplifier I6 through a coupling condenser 38 :and the cathode of the rectifier device [9 has applied thereto a positive bias from a source of unidirectional potential 40. Additionally, a rec-. tifier device 391s connected, with polarity opposite toxthat of the device l9, across the series circuitv comprising the device I 9, the resistor 20, and the battery 40,; The value of thebattery 40 is such as'to' cause the rectifier device Hi to clip at the amplitude level a, b, Fig. 2. The operation of this modified formof clipping system is otherwise essentially similar to that of the clipping system of Fig. 1, except that the rectifier device 39 prevents any charge from building up across the rectifier device [9 due to the rectifying action thereof and also aids in'increasing the decrement of the tuned circuit l1, l8.

Fig. 4'is a circuit diagram of a modification of a portion of the harmonic-frequency generator of Fig. 1, similar circuit elements being desig nated by similar reference numerals. The source H] of. primary oscillations of Fig.- 1 is here shown in more detailed form as comprised of a crystal? controlled oscillator 4|. The oscillator 4| in.- cludes a vacuum tube 42 having input electrodes coupled in conventional manner to a piezo-electric crystal 43 and havin output electrodes coupled to a tuned output circuit comprising a variable inductor 44 and anode-circuit inherent capicitance, represented by the broken-line condenser 45, the output circuit being resonant at the natural frequency of oscillation of the crystal 43. The oscillator 4| thus comprises means for generating primaryoscillations of a predetermined fundamental frequency equal to the frequency of the crystal 43. The blocking oscillator ll of Fig. l is omitted in the modification of Fig. 4 and the output of the oscillator 4| is coupled directly through a condenser 46 to the input circuit of the amplifier 6 which includes a grid leak 46, the time constant of the condenser 46 and grid leak 46' beingsufiiciently long that the amplifier I6 is biased beyond cutoff except during a short interval of each positive half-cycle of the oscillations applied thereto from oscillator 4|. Amplifier It thus operates as a clipper stage. The inductor IT in the output circuit of the amplifier I6 is here shown as of the adjustable type and it will be understood that it is tuned with capacitance Hi to a frequency much higher than the frequency of oscillations generated by the oscillator 4|. The diode rectifier type of clipper system l9 of Fig. 1 is replaced in the arrangement of Fig. 4 by a pentode. type vacuum tube 41, the control grid of which is coupled through a condenser 48 to the output circuit of amplifier It. A grid-leak resistor 49 is connected across the input electrodes of tube 41, the condensor 48and resistor 48 having values proportioned to develop a self-bias for the control grid. of tube 4! in a conventional manner.

, The remainder of the system is identical to that The'primary oscillations are applied to the amplifier or clipper I6 and the anode current thereof occurs in short pulses of the period of the primary oscillations, as represented by curve E. These current pulses shock-excite the inductor l1 and develop thereacross periodically recurrent damped trains of secondary oscillations of frequency higher than that of the primary oscillations, as represented by curve F. These secondary oscillations are applied to tube 41 which acts as a conventional peak rectifier, developing across the resistor 49 a negative self-bias of such value that only the peak of the first positive half-cycle of each train of damped oscillations is repeated by tube 41. There is thus derived in the output circuit of vacuum tube 47 a periodic potential effectively of pulse-wave form, as represented by curve G. This periodic potential corresponds to that of curve D of Fig. 2, is of a similar wave form, and has similar characteristics. eration of this modified form of the invention is otherwise essentially similar to that of Fig. 1 and will not be repeated here.

As illustrative of a specific embodiment of the invention of the type shown in Fig. 4, the following circuit constants have been found satisfactory:

The op- Vacuum Tubes 42 and I6 Type 6807 Vacuum Tube 41 Type 6SJ7 Inductor 44 millihenries 2.82 Inductor l'l microhenries 253 Resistor 1 (approximately) ohms 30,000 Resistor 46' megohm 1.0 Resistor 49 do 1 Condenser 46 microfarad 0.002 Condenser 48 do 0.002 Source +13 volts 250 Source +Sc do 250 Frequency of primary oscillations v kilocycles 400 Natural frequency of inductor I! megacycles 2 Fig. 6 is a circuit diagram representing a modified form of harmonic-frequency generator of the invention essentially similar to that of Fig. 1. In this arrangement, the means for generating primary and secondary oscillations is comprised by a single vacuum tube 50. trodes of vacuum tube 50 are coupled in conventional manner to a piezo-electric crystal device 5! and the screen grid of tube 50 is used as an anode element and connected in a first anode circuit comprising a voltage-dropping resistor 52 and shunt-connected condenser 53, a paralleltuned circuit 54, and a source of energizing potential indicated as +13. The tuned circuit 54 is resonant at the natural period of oscillation of the crystal 5|. The anode of tube 50 is included in a second anode circuit comprising a paralleltuned circuit 55 and is energized from the source of space current +B through the tuned circuits 54 and 55 in series. The tuned circuit 55 is resonant at a frequency which is a harmonic of, and therefore higher than that of, the tuned circuit 54, preferably the fourth harmonic of the latter. The oscillations developed in the tuned circuits 54 and 55 arecombined in the anode circuit of tube 50 and are applied through a condenser 56 to a clipper system comprising a pentode vacuum tube 51, the tube 51 having a grid-leak resistor 58 and acting as a conventional peak rectifier. The output circuit of tube 51 includes an inductor 59 having connected in shunt The input electherewith by selective operation of a switch 60 any one of a plurality of condensers til-64, in-' elusive, which tune the inductor 59 to predetermined harmonic-frequency components. The output circuit terminals 65, 66 of the generator are coupled across the inductor 59.

Considering now the operation of this modification of the invention and referring to the curves of Fig. 7, broken-line curve H represents the wave form of the primary oscillations of fundamental frequency developed across the tuned circuit 54, this voltage being fed back principally through the control grid to screen capacitance of tube 50 to the input circuit comprising the crystal 5i to maintain oscillation. It may be noted that the resistor 52 is a voltage-dropping resistor used for the purpose of reducing the screen-grid voltage below that of the anode of tube 50, the condenser 53 by-passing the resistor 52 for currents of the fundammental oscillation frequency. The control grid of tube 50 develops in Well-known manner a large self-bias whereby tube 50 operates as a class C oscillator. The screen-grid current of tube 50 consequently is of periodic :pulse-wave form and contains a large number of harmonics including a strong fourth harmonic of the fundamental frequency. The screen and control grids of tube 50 effectively modulate the stream of electrons flowing to the anode and there is thus developed across the tuned circuit 55 fourth-harmonic secondary oscillations. The oscillatory voltages developed across the tuned circuits 54 and 55 are combined in the anode circuit of tube 50 and, when properly phased by adjustment of the relative tuning of circuits 54 and 55, produce a resultant output wave of distorted wave form having a large peak amplitude of given polarity during only one relatively small portion of each cycle, as represented by curve I. These latter oscillations are applied to the clipper tube 51 which is effective, as in the previously described modifications, to develop in the anode circuit of tube 51 a periodic potential wave effectively of pulse-wave form as represented by curve K, this potential having a period. equal to that of the primary oscillations developed across the tuned circuit 54 and being rich in harmonic frequency components. A desired harmonic-frequency component is selected by operation of the switch 60 and applied to the output terminals 65, 66 of the generator.

While it has been stated that the tuned circuit 55 is resonant at the fourth harmonic of the oscillations of fundamental frequency developed across the tuned circuit 54, it will be evident that tuned circuit 55 may be resonated to any suitable harmonic frequency, preferably to a higher frequency. This has the effect of narrowing the duration of the pulses of the periodic potential developed in the output circuit of theclipp'er tube 51 with consequent increase in the range over which the harmonicfrequency components have adequate amplitude to be used directly without additional amplification. The tuning of circuit 55 to the fourth harmonic is desirable, however, for the reason that there is a strong fourth harmonic component in the screen-grid current of tube 50 and for the further reason that the harmonic-frequency components developed by the generator have larger power, the power output of any particular lower harmonic component varying directly with the duration of the pulses of the periodic potential developed in the output circuit of tube 51, whereas the range of harmonic-frequency components of substantial amplitude varies inversely with the duration of the periodic potential pulses.

As illustrative of a specific embodiment of the type shown in Fig. 6, the following circuit constants have been found suitable:

Vacuum tube 50 Type 1Q5GT Vacuum tube 51 Type 1Q5GT Tuned circuit 54 Inductance microhenries 200 Capacitance micromicrofarads 150 Tuned circuit 55 Inductance microhenries" 70 Capacitance micromicrofarads 15 Resistor 52 ohms 100,000 Resistor 58 megohms 4.7 Condenser 53 micromicrofarads 500 Condenser 5.6 do 500 Frequency of primary oscillations kilocycles 800 Frequency of secondary oscillations megacycles 3.2 Source +B volts 135 It will be evident from the above description of the Fig. 6 modification that the electronic coupling, within tube 50, of the tuned circuits 54 and 55 comprise means coupling the primaryoscillation generating means, which is tuned circuit 54, and the secondary oscillation developing means, which is the tuned circuit 55, to synchronize the secondary oscillations with the primary oscillations.

It will be evident from the above description of the invention that a harmonic-frequency generator embodying the invention has the advantage of great simplicity and economy of construction and is capable of delivering a wide range of harmonic-frequency components of appreciable amplitude. The generator has the additional advantages of a high degree of flexibility of its operating characteristics and can be easily initially adjusted for best operation and, when once adjusted, is stable in operation without further adjustment over long periods of operation.

While there have been described what at present are considered to be the preferred embodiments of the invention, it will be understood by those skilled in the art that various changes and modifications may be made herein without departing from the invention, and it is, therefore, aimed in the appended claim to cover all such modifications and changes as fall within the spirit and scope of the invention.

What is claimed is:

A harmonic-frequency generator comprising, means for generating primary oscillations of a predetermined fundamental frequency, means for developing secondary oscillations of higher frequency than said primary oscillations, means for combining said primary and secondary oscillations to derive oscillations of unsymmetrical wave form having a large peak amplitude of given polarity during only one relatively small portion of each cycle, and means for clipping said oscillations of unsymmetrical wave form to derive only said peak-amplitude portion efiectively comprising a periodic wave of pulse-wave form, of period equal to that of said primary oscillations, and rich in harmonic-frequency components.

RUDOLF E. STURM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,908,249 Hund May 9, 1933 2,013,806 Osnos Sept. 10, 1935 2,181,280 Miller Nov. 28, 1939 2,181,568 Kotowski Nov. 28, 1939 2,403,624 Wolff July 9, 1946 FOREIGN PATENTS Number Country Date 824,086 France Nov. 3, 1937 

